1. Field of the Invention
The present invention generally relates to a video tape recorder. More specifically, the present invention is directed to a video tape recorder capable of recording information for a long time and suitable for use as a monitor.
2. Description of the Prior Art
In general, an intermittent recording operation is repeatly performed in a video tape recorder capable of recording information for a long time in order to be useful for a monitoring purpose. In this intermittent recording operation, a recording operation is performed when the video tape is stopped, whereupon, the video tape is fed for a predetermined time interval, and then the recording operation is effected a new a state that this video tape is again stopped. Then, during the reproducing operation, video scenes are reviewed while the tape is continuously fed in the continuous reproducing mode. If there is a video scene to be observed, this video scene may be investigated by sequentially, slowly forwarding or rewinding the tape so that scenes located near the first-mentioned video scene may be observed.
In FIG. 9, there is shown one example of such a long recording time type video tape recorder.
In FIG. 9, reference numeral 1 indicates a rotary head apparatus; reference numeral 2 denotes a tape guide drum; and reference numerals 3A and 3B represent rotary magnetic heads. A magnetic (video) tape 4 is wound on the tape guide drum 2 in an inclined angle over a range of 180.degree..
The magnetic heads 3A and 3B are mutually arranged with an angular interval of 180.degree. therebetween. These heads 3A and 3B are so arranged that each of the magnetic heads is rotatable through one full revolution within 1 frame period (namely, 1/30 seconds in the NTSC color TV system). The heads 3A and 3B are mounted in such a manner, as shown in FIG. 10, that there is provided between the heads 3A and 3B, in the direction of the axis of rotation of the heads a step "h" corresponding to 1 track pitch on the magnetic tape 4, and also the azimuth angles of these heads are different from each other.
To an input terminal 5, a video signal SVin of, e.g., the NTSC color TV system, is supplied from a monitoring video camera (not shown) or the like. This video signal SVi is supplied to a recording process circuit 6. In this recording process circuit 6, such processes as the FM modulation of the luminance signal and the low-frequency range conversion for the color (chrominance) signal are carried out.
A recording video signal outputted from the recording process circuit 6 is supplied via a connection switch 7 to the magnetic heads 3A and 3B. The switch operation of this connection switch 7 is controlled by a system controller 8. As will be described later, the magnetic tape 4 is intermittently driven, or transported by a tape drive device, such as, a capstan 15 driven by a motor 16 under control of the system controller 8, during a recording operation, and a video signal for 1 frame is recorded by the heads 3A and 3B on the tape 4 while the tape drive device is in a stop mode. The above-explained connection switch 7 is turned ON in a time period corresponding to the recording period of the video signal by the heads 3A and 3B, so that the recording video signal outputted from the recording process circuit 6 is furnished via the connection switch 7 to the magnetic heads 3A and 3B.
Referring now to FIG. 11, a description will be provided of a recording operation on the magnetic tape 4. It should be noted that, in this recording operation, only 1-frame of a video signal is recorded on the tape 4 during each of 6-frame interval of the recording video signals outputted from the recording process circuit 6.
FIG. 11A represents a video signal SVin to be supplied to the input terminal 5. With respect to symbols of "Aab" and "Bab" shown in FIG. 11A, symbols "A" and "B" indicate an odd field, respectively and an even field of each of the frames and the symbol "ab" represents a b-frame within an a-th tape feeding interval.
FIG. 11B indicates a drive condition of the tape 4. During a first 1-frame period of one tape feeding interval comprised of 6 frames, the tape 4 remains in a stationary condition, and during the subsequent five frame periods, the tape drive is controlled so that the tape 4 sequentially passes from a stop state to a driven state in which the tape 4 is fed over a distance corresponding to the pitch of two tracks in which one frame is recorded, and finally the tape is restored to the stop state.
Since the recording video signal outputted from the recording process circuit 6 is supplied via the connection switch 7 to the heads 3A and 3B during the first frame within the respective tape feeding interval, during which the tape 4 is in the stationary state, the video signals in the odd and even fields of the first frame are recorded on the tape 4. FIG. 11C indicates video signals actually recorded on the tape 4 during successive periods when the tape is in the stationary state.
It should be understood that two recording tracks are formed on the tape 4 for each tape feeding interval of six frames, and the recording video signals for two fields, is, an odd field and an even field, are recorded by the heads 3A and 3B on these recording tracks.
As described above, the long-time recording operation is available since only a single frame of the video signal is recorded on the tape 4 during each respective 6-interval of the recording video signals outputted from the recording process circuit 6, and a so-called "intermittent recording" operation is carried out.
As previously explained, the drive conditions of the tape 4 are controlled by the system controller 8. The tape 4 is driven at the normal reproducing speed in the continuous reproducing mode, whereas the tape 4 is brought into the stationary state or condition in the still reproducing mode under control of the system controller. It should be noted that, when the adjoining video scenes are reproduced in the still reproducing mode, the tape drive condition is sequentially changed from the stop state, to the drive state and again returned to the stop state, which is similar to the tape drive conditions during the above-explained recording operation, so, when in the drive state, the tape 4 is fed by 2 tracks (namely, 1 frame).
As shown in FIG. 9, a reproducing video signal reproduced from the tape 4 by means of a magnetic heads 3A and 3B is supplied to the reproducing process circuit 9. In this reproducing process circuit 9, such processes as the demodulation of the FM luminance signal from the converted low frequency range and the frequency conversion of the color signal are carried out.
A video signal outputted from the reproducing process circuit 9 is supplied to a fixed terminal at a side "a" of a changing switch 10. The video signal outputted from the reproducing process circuit 9 is A/D-converted by an A/D converter 11 into a digital video signal. The resulting digital video signal is supplied as a write signal to a field memory 12. The data writing/reading operations of the field memory 12 are controlled by the system controller 8.
The digital signal read out from the field memory 12 is D/A-converted by a D/A converter 13 into an analog signal which will then be supplied to a fixed terminal at a side "b" of the changing switch 10. The changing operations by the changing switch 10 are performed under control of the system controller 8 in such a manner that, only when the data stored in the field memory 12 is read out, the slider of the changing switch 10 is connected to the side "b" of this switch 10. In all other cases, the slider of the changing switch 10 is connected to the side "a" thereof. A video signal "SVout" derived from the changing switch 10 is conducted to an output terminal 14.
In FIG. 12, there are represented recording track patterns formed on the tape 4. Symbols T0a, T1a, T2a, - - - , denote recording tracks formed by the head 3A, and symbols T0b, T1b, T2b, - - - , represent recording tracks formed by the head 3B.
In the continuous reproducing mode, since the tape 4 is driven at the normal reproducing speed, the scanning trails LA and LB of the magnetic heads 3A and 3B are included from the recording tracks, so that the magnetic heads 3A and 3B cannot correctly scan the recording tracks. Accordingly, in this example, a tracking control operation is carried out in such a way that the central portions of the recording tracks T0a, T1a, T2a,- - - , are scanned by the head 3A. It should also be noted that during the recording operation, a pilot signal has been recorded together with the recording video signal on the respective recording tracks, and then, the well-know ATF type tracking control is performed with reference to the recorded pilot signal.
As previously described the step "h" corresponding to one track pitch, is provided between the magnetic heads 3A and 3B. When tracking control is carried out in such a manner that the central portions of the recording tracks T0a, T1a, T2a, are scanned by the head 3A, the head 3B scans end portions of the recording tracks T0b, T1b, T2b, - - - .
As a result, although the signal level of the reproduced video signal derived from the head 3A is slightly lowered for both ends of each field during which the tape 4 is scanned by this head 3A, a sufficient signal level of this reproduced video signal can be obtained over the entire period of each field. To the contrary, the reproduced video signal derived from the head 3B cannot have a sufficient signal level over the entire period of each field during which the tape 4 is scanned by this head 3B. In FIG. 13A which represents video signals reproduced from the magnetic heads 3A and 3B, symbols Sa1, Sa2, Sa3, - - - , indicate the video signals reproduced from the head 3A, whereas symbols Sb1, Sb2, Sb3, - - - , represent the video signals reproduced from the head 3B.
As explained above, the video signals reproduced from the magnetic head 3B cannot be obtained at sufficient signal levels. If the video signals outputted from the reproducing process circuit 9 would be directly supplied through fixed contact "a" of switch 10 to the output terminal 14, the image quality of the reproduced video screen would be considerably deteriorated due to noise.
Accordingly, in the example of FIG. 9, as shown in FIG. 13B, the video signal outputted from the reproducing process circuit 9 is written into the field memory 12 within each field period during which the tape 4 is scanned by the head 3A, whereas the video signal for the one field which has been written in the previous field period is read out from the field memory 12 within the succeeding field period during which the tape 4 is scanned by the head 3B.
Since the changing switch 10 is normally connected to the side "a" and is connected to the side "b" only when the contents of the field memory 12 are being read therefrom, only the video signal outputted from the reproducing process circuit 9 in response to the reproduced video signal derived from the head 3A, is passed through the switch 10 and then is conducted from the output terminal 14. In FIG. 13C, symbols Sa1', Sa2', Sa3' represent video signals derived from the reproducing process circuit 9 in accordance with the reproduced video signals Sa1, Sa2, Sa3 from the head 3A.
As previously explained, during a playback operation in the continuous reproducing mode, since only the video signal outputted from the reproducing circuit 9, which corresponds to the reproduced video signal a derived from the head 3A, and hence having a sufficient signal level, is supplied to the output terminal 14, deterioration of the image quality of the reproduced video screen is deteriorated due to noise can be prevented.
In the conventional video tape recorder FIG. 9, both of a frame reproduction and a field reproduction can be carried out in a still reproduction mode.
In the still reproduction mode, as illustrated in FIG. 14, since the tape 4 is brought into the stationary condition, the incline of the scanning or traces by the heads 3A and 3B is coincident with that of the recording tracks, so that these heads 3A and 3B can correctly scan the recording tracks. Further, since the tape 4 is brought into the stationary state and the step "h" corresponding to one track pitch is provided between the heads 3A and 3B, when tracking control is performed such that the recording tracks T0a, T1a, T2a, - - - , are scanned by the head 3A, then the head 3B is also positioned so as to scan the relevant recording tracks T0b, T1b, T2b, - - - .
When the frame reproduction is carried out in the still reproduction mode, the changing switch 10 remains connected at the side "a", and the video signals Sa' and Sb' outputted from the reproducing process circuit 9 in response to the video signals Sa, Sb reproduced from the heads 3A and 3B, are directly conducted to the output terminal 14.
FIGS. 15A and 15B, represent the reproduced video signals Sa, Sb output from the heads 3A and 3B and FIG. 15C, shows a video signal "SVout" outputted from the output terminal 14, and in which the video signals Sa' and Sb' for the odd field and the even field are alternately arranged.
When the field reproduction is performed in the still reproduction mode, both the writing operation and the reading operation for the field memory 12 are controlled, by the system controller 8, and also the switching operation of the changing switch 10 is similarly controlled. Thus, the video signals Sa' or Sb' derived from the reproducing process circuit 9 in correspondence to the reproduced video signals Sa or Sb from the heads 3A or 3B, respectively, are conducted via the changing switch 10 to the output terminal 14.
In other words, as shown in FIGS. 16A and 16B, the video signals Sa and Sb are reproduced by the heads 3A and 3B. In the field period during which the reproduced video signal Sa is outputted, the changing switch 10 is connected to the side "a" so that the respective video signal Sa' is supplied directly through switch 10 to the output terminal 14. The video signal Sa' outputted from the reproducing process circuit 9 in accordance with this reproduced video signal Sa is also written into the field memory 12 (FIG. 16c). On the other hand, in the field period during which the reproduced video signal Sb is outputted from the head 3B, the changing switch 10 is connected to the side "b", and then the video signal Sa' which had been written in the field memory 12 during the preceding field is read out from this field memory 12. As a result, a video signal SVout constructed of only the video signal Sa' of the field reproduced by the head 3A is outputted to the output terminal 14, indicated in FIG. 16D.
It should be noted that if the video signal Sb1 outputted from the reproducing process circuit 9 in response to the reproduced video signal Sb outputted from the head 3B is written into the field memory 12 and the switching operation of the changing switch 10 is reversed, a video signal SVout composed of only the video signal Sb' in the field reproduced by the head 3B would be obtained from the output terminal 14.
When there is a large movement in an image, a blurring phenomenon of the image may be mitigated by executing the above-explained field reproduction.
On the other hand, in order to realize the frame recording/reproducing operations while the tape 4 is under in the stationary state with the rotary head apparatus 1 shown in FIG. 9, the step "h" corresponding to one track pitch on the tape 4 has to be formed between the heads 3A and 3B.
However, this step "h" is approximately 17 to 20 microns at the most. Accordingly, since very high machining precision is required to form such a step "h", the manufacturing cost of the rotary head apparatus 1 becomes high.
There is another problem in that when the rotary head apparatus 1 is constructed with the step between the heads 3A and 3B, the normal video signal recording operation cannot be performed. In other words, it is not possible with the apparatus of FIG. 9 to change from the intermittent recording operation to a continuous recording operation as is often required in the security market in an emergency case.